1. Field of the Invention
The present invention relates to membrane pumps, and in particular, but not exclusively, to a membrane pump for a dialysis machine.
2. State of the Art
Membrane pumps are used in a number of medical applications, for example in hemodialysis for the extracorporeal circulation of blood and the preparation and delivery of dialysate to and from the dialyser. Commonly, the membrane covers a dome shaped pump chamber into which the membrane is actuated to draw fluid into, and pump the fluid from, the chamber.
A known pump is described with reference to FIG. 1 below. This form of pump has a number of drawbacks as follows.
The sharp angle between the concave recess of the drive chamber and the flat portion of the pump to which the membrane is attached causes high stresses in the membrane as it folds and stretches over the edge of the flat portion and into the recess upon actuation. This can lead to reduced life of the membrane.
Furthermore, the sharp angle between the flat portion and the recess can also cause the membrane to fold over the edge leaving a dead spot between the membrane and the recess wall where the membrane has not been actuated sufficiently to touch the chamber wall and expel fluid therebetween. This can lead to pumping inaccuracies and even to blood damage where the blood pools in this area for an unacceptable length of time.
The dome shaped profile of the pump chamber also presents a number of problems. The substantially hemispherical shape means that the membrane descends rapidly into the chamber upon actuation. Thus a large volume of fluid is pumped at the beginning of the stroke compared to the end of the stroke. Thus the volume of dispensed fluid can be difficult to control on the length of stroke. This can also lead to a hammer action of the pump which can lead to pumping inaccuracies and potential blood damage.